iOS: how to add length of a NSData as its two bytes header?

Question

I have a NSData and I would like to append its length in its header as hex digits. I am able to do this using following code:

unsigned int len = [data length];

NSMutableData *sendData = [[NSMutableData alloc] initWithBytes:&len length:2];
[sendData appendData:data];

result from above code for len = 5 is "05 00" but I want "00 05" instead. Does anyone know how to do that?

Header is always going to be of length 2.

Thanks,

This likely has to do with the endianness, try `unsigned int len = CFSwapInt16BigToHost([data length]);` — Scott Berrevoets, Mar 08 '13 at 21:45
There's an article referring to endian swapping at http://www.gamedev.net/page/resources/_/technical/game-programming/writing-endian-independent-code-in-c-r2091 — Richard Brown, Mar 08 '13 at 21:46
@Scott Shouldn't that be `CFSwapInt16HostToBig`? The goal is to go from the current 'host` endian to the desired big endian. — rmaddy, Mar 09 '13 at 00:56
BTW - here's a good example of big versus little endian: http://www.chucklecloud.com/chuckles/1270 — rmaddy, Mar 09 '13 at 00:59

Jean · Accepted Answer · 2013-03-09T12:16:23.037

2

The code below addumes (as you do) that the length is less than 65536 (two bytes). So you need to use:

uint16_t len = CFSwapInt16HostToBig([data length]);
NSMutableData *sendData = [[NSMutableData alloc] initWithBytes:&len length:2];
[sendData appendData:data];

The list of available function are described in de document below in the apple developer library:

Byte-Order Utilities Reference

edited Mar 09 '13 at 12:16

answered Mar 08 '13 at 22:32

Jean

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Your swap is backwards. You want to go from `host` to big endian. `Host` always represents what you have on the current device. You should also assign the raw length to a `uint16_t` variable first, then swap that variable. – rmaddy Mar 09 '13 at 00:53
2

My mistake. Most of the time I need to do the reverse, so I accepted the auto-completion in Xcode before reading the full text… Thanks for seeing it. – Jean Mar 09 '13 at 12:17

iOS: how to add length of a NSData as its two bytes header?

1 Answers1